If the exhaust manifold bolts are broken, you can fix this your self depending on your mechanical abilities. Basically, the manifolds will have to come off, holes will have to be drilled into the stubs and pull out the stubs. Then buy new bolts and nuts, reassemble and you are done. I had to purchase a 90-degree drill and had my 3 ton floor jack handy. I have a '73 450 SEL that this needed to be done.
I am not sure how much something like this would cost. This is not as big of a problem as it may sound. I love my car and I am willing to fork $5000 for a rebuilt engine. Is it worth time and money to you?
How did they break them is the question you should ask. It may indicate a larger problem ranging from loose exhaust, low suspension or even shifting engine.
Then again, someone may have simply over tightened which is easy to do on heat treated metal.